UV map for cuboid produces islands of same size
It sounds like you have scaled your mesh. Unwrap operations should always be done with the object scale set to 1 (to avoid problems like yours). 

In object mode if you select the object and look at the scale in the properties region, you will see that it is not all 1.

Press CtrlA > Scale, to apply the scale.
Then in edit mode re-unwrap the mesh, and it will have the proper scale.

As an aside, if you were to use the Unwrap operation, blender would warn you with this message.


  Object has non-uniform scale, unwrap will operate on a non-scaled version of the mesh.